1. Filed of the Invention
The present invention relates to an apparatus for estimating tire air pressure, and specifically to a method and apparatus for estimating the state of air pressure of a tire or each of tires based on wheel speed.
2. Description of the Related Art
As an apparatus for estimating the state of air pressure of a tire, a technique has conventionally been known wherein a wheel speed signal including a vibration frequency component of a tire at the time of running of a vehicle is subjected to frequency analysis and a resonance frequency of the tire is extracted therefrom, and, based on the resonance frequency, the state of air pressure of the tire is detected (see Japanese Patent Registration No. 2,836,652 and the like).
A resonance frequency to be set in the technique for estimating air pressure based on the resonance frequency of a tire is generally in the range from about 30 to 50 Hz. However, this technique has a problem that there are cases in which the air pressure of a tire cannot be accurately estimated depending on the state in which a vehicle is placed.
For example, when the speed of a vehicle is set in a range from a low speed to a medium speed when the vehicle is running in an urban area, the air pressure of a tire can be estimated with high accuracy at approximately 30 to 50 Hz in the above-described range of resonance frequencies. However, when the speed of a vehicle is set in a range of high speed, a tire vibration phenomenon tends not to occur, and a power spectral level of resonance frequency is lowered. As a result, estimation accuracy of the air pressure of the tire is reduced.
On the other hand, Japanese Patent Application Laid-Open (JP-A) No. 9-2031 discloses a technique in which, when the speed of a vehicle is low, the air pressure of a tire is estimated based on a spring constant of the tire, which is an index having a high correlation with the above-described resonance frequency, and, when the speed of the vehicle is high, the air pressure of the tire is estimated based on dynamic load radius of the tire, thereby allowing highly accurate estimation of the air pressure of the tire irrespective of the speed of the vehicle.
However, in the technique in which the state of air pressure of a tire is estimated based on the dynamic load radius of a tire as disclosed in the above JP-A No. 9-2031, there exists a drawback in that, when a vehicle is running at a high speed, slipping of driving wheels may occur or the diameter of the tire may increase by centrifugal force, that is, so-called xe2x80x9ctread liftxe2x80x9d may occur, with the result that highly accurate estimation cannot be made.
The present invention has been achieved so as to solve the above-described problems, and an object of the present invention is to provide an apparatus for estimation of tire air pressure in which the state of air pressure of a tire can be estimated with high accuracy in a range of vehicle speeds from a low travel speed to a high travel speed.
In order to achieve the above-described object, an apparatus for estimating tire air pressure according to a first aspect of the present invention includes: a wheel speed detection component for detecting wheel speed of each of a front wheel and a rear wheel; a characteristic amount calculation component for calculating a characteristic amount of a frequency characteristic of each of the wheel speeds detected by the wheel speed detection component; a difference-value calculation component for calculating a value of difference which represents a degree of difference between a characteristic amount of the front wheel and a characteristic amount of the rear wheel, which characteristic amounts are calculated by the characteristic amount calculation component; and a state determination component for determining a state of air pressure of a tire based on the value of difference calculated by the difference-value calculation component.
In the apparatus for estimating tire air pressure according to the first aspect of the present invention, the wheel speed of each of the front wheel and the rear wheel is detected by the wheel speed detection component, and the characteristic amount of a frequency characteristic of each of the detected wheel speeds is calculated by the characteristic amount calculation component. The characteristic amount comprises an amplitude of a predetermined frequency of the detected wheel speed or frequencies which are mostly included in a predetermined frequency range.
Further, in the first aspect of the present invention, the value of difference which represents the degree of difference between the characteristic amount of the front wheel and the characteristic amount of the rear wheel, which characteristic amounts are calculated by the above-described characteristic amount calculation component, is calculated by the difference-value calculation component. Based on the calculated value of difference, the state of air pressure of a tire is determined by the state determination component.
In other words, in the present invention, the state of air pressure of a tire is determined based on the degree of difference in the characteristic amounts of frequency characteristics between the wheel speeds of the front wheel and the rear wheel. For this reason, even if power spectral level of a resonance frequency of the tire is lowered or the above-described tread lift occurs due to an increase in the travel speed of the vehicle to a high speed, the power spectral level is lowered or the tread lift occurs substantially similarly in all of wheels mounted to the vehicle. As a result, the state of air pressure of a tire can be estimated with high accuracy.
Further, in the present invention, the state of air pressure of a tire is determined based on the difference between the respective characteristic amounts of front and rear wheels. Because the front and rear wheels are provided at the same side, that is, at a left side or right side of the vehicle to which the front wheel and the rear wheel are applied in the present invention, even when the vehicle is running on a road surface having different states between a region on which the right wheels are running, and a region on which the left wheels are running (so-called straddle running), the state of air pressure of a tire can be estimated with high accuracy.
As described above, according to the first aspect of the present invention, the wheel speed of each of the front wheel and the rear wheel is detected, the characteristic amount of the frequency characteristic of each of the detected wheel speeds is calculated, the value of difference which represents the degree of difference between the calculated characteristic amount of the front wheel and the calculated characteristic amount of the rear wheel is calculated, and further, based on the calculated value of difference, the state of air pressure of a tire is determined. Accordingly, the state of air pressure of a tire can be estimated with high accuracy in the range of vehicle speeds from a low speed of travel to a high speed of travel.
The difference-value calculation component in the first aspect of the present invention can be provided so as to calculate, as the value of difference, a difference or ratio between the characteristic amount of the front wheel and the characteristic amount of the rear wheel.
Further, the state determination component in the first aspect of the present invention can be provided so as to make a determination that air pressure of a tire has been reduced when the degree of difference between the characteristic amount of the front wheel and the characteristic amount of the rear wheel, represented by the value of difference, is a predetermined value or more.
Moreover, the state determination component in the first aspect of the present invention may be provided so as to make a determination that air pressure of a tire has been reduced when a degree of difference between the value of difference and a predetermined reference value is a predetermined value or more.
In order to achieve the above-described object, there is also provided an apparatus for estimating tire air pressure according to a second aspect of the present invention, including: a wheel speed detection component for detecting wheel speed of each of a right-hand front wheel, a right-hand rear wheel, a left-hand front wheel and a left-hand rear wheel; a characteristic amount calculation component for calculating a characteristic amount of a frequency characteristic of each of the wheel speeds detected by the wheel speed detection component; a right-hand wheels difference-value calculation component for calculating a value of difference between right-hand wheels, which represents a degree of difference between a characteristic amount of the right-hand front wheel and a characteristic amount of the right-hand rear wheel, which characteristic amounts are calculated by the characteristic amount calculation component; a left-hand wheels difference-value calculation component for calculating a value of difference between left-hand wheels, which represents a degree of difference between a characteristic amount of the left-hand front wheel and a characteristic amount of the left-hand rear wheel, the characteristic amounts being calculated by the characteristic amount calculation component; and a state determination component for determining a state of air pressure of a tire based on the value of difference between right-hand wheels and the value of difference between left-hand wheels.
In the apparatus for estimating tire air pressure according to the second aspect of the present invention, the wheel speed of each of the right-hand front wheel, the right-hand rear wheel, the left-hand front wheel and the left-hand rear wheel is detected by the wheel speed detection component, and the characteristic amount of the frequency characteristic of each of the detected wheel speeds is calculated by the characteristic amount calculation component.
Further, according to the second aspect of the present invention, the value of difference between right-hand wheels, which represents the degree of difference between the characteristic amount of the right-hand front wheel and the characteristic amount of the right-hand rear wheel, which characteristic amounts are calculated by the above-described characteristic amount calculation component, is calculated by the calculation component for a value of difference between right-hand wheels. The value of difference between left-hand wheels, which represents the degree of difference between the characteristic amount of the left-hand front wheel and the characteristic amount of the left-hand rear wheel, which characteristic amounts are calculated by the above-described characteristic amount calculation component, is calculated by the calculation component for a value of difference between left-hand wheels. Based on the obtained value of difference between right-hand wheels and value of difference between left-hand wheels, the state of air pressure of a tire can be determined by the state determination component.
In other words, in the present invention, the state of air pressure of a tire is determined based on the degree of difference in the characteristic amounts of frequency characteristics between the wheel speeds of the right-hand front wheel and the right-hand rear wheel, and the degree of difference in the characteristic amount of frequency characteristics between the wheel speeds of the left-hand front wheel and the left-hand rear wheel. For this reason, even if a power spectral level of resonance frequency of a tire is lowered or the above-described tread lift occurs due to an increase in the travel speed of the vehicle to a high speed, the power spectral level is lowered or the tread lift occurs substantially similarly in all of wheels mounted to the vehicle. As a result, the state of air pressure of a tire can be estimated with high accuracy.
Further, in the present invention, the state of air pressure of a tire is determined based on the difference between the characteristic amounts of front and rear wheels located at the same side of the vehicle. Accordingly, even in the case of straddle running of the vehicle, the state of air pressure of a tire can be estimated with high accuracy.
As described above, according to the second aspect of the present invention, the wheel speed of each of the right-hand front wheel, the right-hand rear wheel, the left-hand front wheel and the left-hand rear wheel is detected, the characteristic amount of the frequency characteristic of each of the detected wheel speeds is calculated, the value of difference between right-hand wheels, which represents the degree of difference between the characteristic amount of the right-hand front wheel and the characteristic amount of the right-hand rear wheel, is calculated, and the value of difference between left-hand wheels, which represents the degree of difference between the characteristic amount of the left-hand front wheel and the characteristic amount of the left-hand rear wheel, is calculated. Further, the state of air pressure of a tire is determined based on the obtained value of difference between right-hand wheels and value of difference between left-hand wheels. Accordingly, the state of air pressure of a tire can be estimated with high accuracy in the range of vehicle speeds from a low travel speed to a high travel speed.
The calculation component for a value of difference between right-hand wheels in the second aspect of the present invention can be provided so as to calculate, as the value of difference between right-hand wheels, a difference or ratio between the characteristic amounts of the right-hand front wheel and the right-hand rear wheel. The calculation component for a value of difference between left-hand wheels can be provided so as to calculate, as the value of difference between left-hand wheels, a difference or ratio between the characteristic amounts of the left-hand front wheel and the left-hand rear wheel.
The state determination component in the second aspect of the present invention can be provided so as to calculate, as a value of difference between right and left, a difference or ratio between the value of difference between right-hand wheels and the value of difference between left-hand wheels, and, when the degree of difference between the value of difference between right-hand wheels, and the value of difference between left-hand wheels, represented by the value of difference between right and left, is a predetermined value or more, the state determination component can be provided so as to make a determination that the air pressure of a tire has been reduced.
Further, in the state determination component according to the second aspect of the present invention, the difference or ratio between the value of difference between right-hand wheels and the value of difference between left-hand wheels is calculated as the value of difference between right and left, and, when the degree of difference between the value of difference between right and left is a predetermined value or more, it is determined that the air pressure of a tire has been reduced.
Moreover, the characteristic amount of the frequency characteristic of a wheel speed in the second aspect of the present invention can be made an amount related to power in a predetermined frequency range of the wheel speed.
In accordance with a third aspect of the present invention, there is provided a method for estimating tire air pressure, comprising the steps of: (a) detecting wheel speed of each of a front wheel and a rear wheel; (b) calculating a characteristic amount of a frequency characteristic of each of the wheel speeds detected in the step (a); (c) calculating a value of difference which represents a degree of difference between a characteristic amount of the front wheel and a characteristic amount of the rear wheel, which characteristic amounts are calculated in the step (b); and (d) determining a state of air pressure of a tire based on the value of difference calculated in the step (c).
In accordance with a fourth aspect of the present invention, there is provided a method for estimating tire air pressure, comprising the steps of: (a) detecting wheel speed of each of a right-hand front wheel, a right-hand rear wheel, a left-hand front wheel and a left-hand rear wheel; (b) calculating a characteristic amount of a frequency characteristic of each of the wheel speeds detected in the step (a); (c) calculating a value of difference between right-hand wheels, which represents a degree of difference between a characteristic amount of the right-hand front wheel and a characteristic amount of the right-hand rear wheel, which characteristic amounts are calculated in the step (b); (d) calculating a value of difference between left-hand wheels, which represents a degree of difference between the characteristic amount of the left-hand front wheel and the characteristic amount of the left-hand rear wheel, which characteristic amounts are calculated in the step (b); and (e) determining a state of air pressure of a tire based on the value of difference between right-hand wheels and the value of difference between left-hand wheels.